The present invention relates to a method of producing stacks of sheets printed with individual prints, in particular stacks of securities sheets, such as banknotes, from stacks of unprinted sheets, the unprinted sheets running into a printing machine one after another in order to be provided with the individual prints.
The present invention likewise relates to a printing machine for the implementation of the method according to the invention.
Methods of producing stacks of securities sheets and machines for the implementation of these methods are known from the current prior art. For example, the patent EP 0 563 007, the content of which is incorporated by reference, discloses a copper printing machine of modular construction which, in particular, contains an impression cylinder on which the sheets to be printed are firmly held, and a plate cylinder which in turn carries copper printing plates into which the printing motifs transferred to the sheets of the impression cylinder are engraved. In the known machine, the inking of the printing plates is carried out both on an indirect route by means of an ink collecting cylinder, which collects the ink from a plurality of ink ductors and subsequently transfers it to the plates of the plate cylinder, and on a direct route via an ink applicator cylinder, which is laid directly on the printing plates.
The method of producing the stack of printed sheets comprises, in particular, the transfer of the unprinted sheets from the feed stack to the impression cylinder, then the actual printing of the sheets with the motifs, the motifs being formed by means of the inked plates of the plate cylinder, and finally the onward transport of the printed sheets to a delivery stack.
One difficulty which occurs in certain printing machines, in particular in machines in which the copper printing process is applied, is the problem of ink drying. In such a printing process, the amount of ink actually applied to each sheet is rather large, and it is absolutely necessary for the ink to be dried before the sheets are stacked in the delivery of the machine. This is because, if the ink has had no time to dry completely before being stacked, the sheets then soil one another and can have faults in some prints or then even become completely unusable. Faults of this type have to be reduced to the maximum extent possible, in particular on account of the costs of the sheets used for printing securities and also because of the problems which subsequently occur in the phase of numbering the individual securities, which is carried out consecutively. Each security which has a fault has to be replaced by an individual fault-free security.
Thus, in the current prior art, the proposal has been made to equip the printing machines with an ink drying device. A device of this type is described, for example, by EP 0 039 526, the content of which is incorporated by reference. It primarily comprises a chain gripper system, which transports the freshly printed sheets at constant intervals to a heat source. One of the objects of this system was to save the insertion operation, as it is known, which consisted in a white sheet being inserted between each printed sheet, in order to prevent the printed sheets sticking to one another.
In the case of other machines of the known prior art, the two systems mentioned above (namely, a drying system and a system inserting intermediate sheets) have also been combined. In a machine known from the current prior art, as illustrated in FIG. 1 of this application, drying devices such as lamps or other equivalent means are provided, which permit the printed sheets to be dried before being stacked. After the sheets have been printed, they are conveyed through an output device which, for example, contains a chain gripper system, and during the passage through this, output device, heat sources act on the sheets and the ink is dried.
In order to improve this machine, it was provided with a special feed device for intermediate sheets beside the delivery stack of the printed sheets. With the aid of this feed means, between each printed sheet an intermediate sheet of paper is placed, with which the printed sheets are better protected from one another. The paper used for the intermediate sheets is also cheaper than the paper used for the securities sheets, so that this principle is very advantageous.
The use of intermediate sheets also constitutes a practical solution, in particular for small machines or when there is little space and it is therefore not possible to install an appropriate drying device, as described above. Nevertheless, in the case of small machines it is also complicated to insert intermediate sheets between each printed sheet, since the production speed is high and the paths traced by each printed sheet are short.
The object of the invention is to improve the known methods and machines.
In particular, the invention has the object of proposing a method with which stacks of printed sheets can be formed and, at the same time, the risk of soiling of the stacked sheets can be reduced to the maximum extent.
The object of the invention is also to propose a printing machine, in particular a sheet-fed printing machine, which permits the use of the method.
This object is achieved by the method and the machine which are defined in the independent claims.
Particular types of embodiment of the method and of the machine according to the invention are defined by the independent claims.